Recently, various types of flat panel display devices have come into the spotlight as display devices that can substitute for cathode ray tubes, such flat panel display devices including liquid crystal display (LCD) devices, field emission display (FED) devices, plasma display panel (PDP) devices and electroluminescence (EL) devices.
In a conventional process for fabricating a flat panel display device, a patterning step is generally performed by photolithography. A photolithographic process has a series of photographic processing steps including coating a photoresist, aligning a mask, exposure, development and stripping. Such photolithographic processes are problematic in that they require a long period of processing time, consume a photoresist and a stripping solution for removing a photoresist pattern, and need expensive systems such as an exposure system. Particularly, as a substrate increases in size and a pattern formed on the substrate decreases in size, a more expensive exposure system is required. Moreover, it is difficult to control a pitch precision and electrode width in such processes.
Meanwhile, an ink jet process for forming an electrode has been proposed in view of the realization of a fine line width, low material loss and simple processing steps. Such ink jet patterning processes have come into the spotlight as direct printing processes applicable to various fields in addition to the field of flat panel display devices.
An ink jet process allows direct patterning of a desired pattern on a substrate by using an ink jet head having a plurality of fine nozzles. Thus, an ink jet process includes a decreased number of processing steps, is more cost efficient in equipment investment, and is more flexible to variations in a pattern, as compared to a photolithographic process. Since an ink jet process inherently does not allow the use of high-viscosity paste, it is necessary to use low-viscosity conductive ink including nanometer-scaled fine particles.
To form an electrode pattern via an ink jet printing process, first, metallic ink comprising a solvent, conductive metal particles, a dispersant and additives is jetted from an ink jet nozzle to print a pattern. Then, heat treatment is carried out to remove the solvent and the dispersant and to allow the remaining metal particles to be bound to each other.
Herein, a metallic pattern formed via an ink jet printing process shows higher conductivity as the metal solid content in the ink increases, as the thickness of a printed metal line increases, and as the organic residue remaining after the heat treatment decreases.
Meanwhile, metal nanoparticles may be prepared by a so-called polyol process. In the polyol process, an alcohol compound having a high boiling point is introduced not only as a reducing agent for cations but also as a solvent. For example, it is possible to obtain Ag nanoparticles by reducing a solution of silver nitrate with ethylene glycol at a temperature of 150□, and a surfactant such as PVP (polyvinyl pyrrolidone) may be used to stabilize the surfaces of such reduced Ag nanoparticles. At this time, to facilitate the reaction, the surfactant is generally used in an excessive amount greater than such amount as may be required to stabilize the surfaces of the metal particles.